1. Field of the Invention.
The present invention, generally, relates to a device that is useful for endoprosthesis purposes and is commonly identified as a stent, and more particularly, it relates to a new and improved method of fabricating a stent.
A stent is particularly useful in the medical field of angioplasty involving the reconstruction of vessels that carry blood in both humans and animals. The stent is used to maintain such blood vessels, structurally, in a clear and open condition.
2. State of the Art.
The methods used today to fabricate stents are difficult to perform, are difficult to form stents with consistent structural torsional and radial stiffness and involve high costs. They are formed of tantalum or stainless steel in complex configurations that can produce a work-hardened crystallization in the metal.
U.S. Pat. No. 5,370,683 to Fontaine describes a stent formed of a single filament wrapped around a mandril with a series of U-shaped bends.
U.S. Pat. No. 5,304,200 to Spaulding describes a method of making stents involving winding an elongated strand forming a helix like structure with the ends welded to an adjacent section.
U.S. Pat. No. 5,217,483 to Tower describes a stent arranged to have U-shaped sections formed in a continuous wire with two ends and with the ends attached together to prevent axial expansion.
For a stent to achieve maximum usefulness, it must be flexible in a bending mode during insertion, and it must exhibit stiffness in both torsional and radial modes in order to provide support. To fabricate stents today, wire is fed continuously from a spool and is formed into a generally sinusoidal configuration.
Then, the wire in this sinusoidal configuration is wound around a mandrel in order to produce a helical arrangement. Next, the crests and troughs in this helical arrangement are pressed together so that they touch at this point, and they are welded to provide the required supporting structure.
The stents today are formed into the required configuration to permit a high level of plastic deformation to be achieved during their use. However, the bending and other deformations of the wire followed by the heating and cooling encountered during the welding produces a condition within the metal wire known as work hardening, which lessens much of the wire's ability to provide support in use.
During use, a stent is in a compressed condition, first. Then, a deflated angioplasty balloon is fed inside the compressed stent. This assembly is inserted into a patient's blood vessel, usually an artery, and moved into position. The balloon is inflated to enlarge the stent to a desired diameter, after which the balloon is removed.
The stent within an artery, or within any other type of vessel, is exposed to repetitive flexing as a part of a circulatory system, both from the systolic and the diastolic variations in blood pressure and from variations in movement of a body. Such loading and unloading of a metallic article can produce further work-hardening of the metal, causing premature failure of support.
Methods of fabrication of stents in the past have produced an aspect of this work-hardening caused by the crystallization within the metallic wire, from the heating and cooling cycles during the prior fabrication processes. Moreover, a stent fabrication method is needed that permits more consistency in stiffness.